LinkedList

初始化

构造方法是个无参方法 + 有初始化数据集合参数的方法

public LinkedList() {
}
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

内部有一个头节点、一个尾节点，以及一个 size 变量记录长度，每个节点记录了后续节点、前缀节点，双向链表

private static class Node<E> {
    E item;
    Node<E> next;
    Node<E> prev;
    Node(Node<E> prev, E element, Node<E> next) {
        this.item = element;
        this.next = next;
        this.prev = prev;
    }
}
transient int size = 0;
transient Node<E> first;
transient Node<E> last;

size()

直接返回 size 变量值

isEmpty()

判断 size == 0

contains(Object o)

判断 indexOf(o) != -1，indexOf(o) 逻辑就是简单的链表遍历

public int indexOf(Object o) {
    int index = 0;
    if (o == null) {
        for (Node<E> x = first; x != null; x = x.next) {
            if (x.item == null)
                return index;
            index++;
        }
    } else {
        for (Node<E> x = first; x != null; x = x.next) {
            if (o.equals(x.item))
                return index;
            index++;
        }
    }
    return -1;
}

add(E e)

调用添加到链表末位方法 linkLast

void linkLast(E e) {
    final Node<E> l = last;
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null)
        first = newNode;
    else
        l.next = newNode;
    size++;
    modCount++;
}

正常添加节点操作，只是判断了下链表是否空的，空的话头节点指向这个节点

remove(Object o)

遍历找到节点后调用 unlink 方法

E unlink(Node<E> x) {
    // assert x != null;
    final E element = x.item;
    final Node<E> next = x.next;
    final Node<E> prev = x.prev;
    if (prev == null) {
        first = next;
    } else {
        prev.next = next;
        x.prev = null;
    }
    if (next == null) {
        last = prev;
    } else {
        next.prev = prev;
        x.next = null;
    }
    x.item = null;
    size--;
    modCount++;
    return element;
}

额外判断当前节点是头节点 & 尾节点的情况

addAll(Collection<? extends E> c);

调用了 addAll(size, c)

public boolean addAll(int index, Collection<? extends E> c) {
    checkPositionIndex(index);
    Object[] a = c.toArray();
    int numNew = a.length;
    if (numNew == 0)
        return false;
    Node<E> pred, succ;
    if (index == size) {
        succ = null;
        pred = last;
    } else {
        succ = node(index);
        pred = succ.prev;
    }
    for (Object o : a) {
        @SuppressWarnings("unchecked") E e = (E) o;
        Node<E> newNode = new Node<>(pred, e, null);
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        pred = newNode;
    }
    if (succ == null) {
        last = pred;
    } else {
        pred.next = succ;
        succ.prev = pred;
    }
    size += numNew;
    modCount++;
    return true;
}

就是构建新链表插入的过程，只不过区分了是末位插入，还是中间插入

addAll(int index, Collection<? extends E> c)

就是上面的分析过程

clear()

public void clear() {
    // Clearing all of the links between nodes is "unnecessary", but:
    // - helps a generational GC if the discarded nodes inhabit
    //   more than one generation
    // - is sure to free memory even if there is a reachable Iterator
    for (Node<E> x = first; x != null; ) {
        Node<E> next = x.next;
        x.item = null;
        x.next = null;
        x.prev = null;
        x = next;
    }
    first = last = null;
    size = 0;
    modCount++;
}

遍历置空，非必须，为了更好的GC，然后首尾节点置空

get(int index)

调用 node(index)，执行查找 index 位置节点的逻辑

Node<E> node(int index) {
    // assert isElementIndex(index);
    if (index < (size >> 1)) {
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    } else {
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}

这里判断了 index 的位置，决定是从前往后遍历，还是从后往前遍历

set(int index, E element)

public E set(int index, E element) {
    Node<E> x = node(index);
    E oldVal = x.item;
    x.item = element;
    return oldVal;
}

也是先查找节点，然后替换数据

add(int index, E element)

根据 index 的位置执行不同的逻辑

public void add(int index, E element) {
    checkPositionIndex(index);
    if (index == size)
        linkLast(element);
    else
        linkBefore(element, node(index));
}

linkLast 插入到末尾

void linkLast(E e) {
    final Node<E> l = last;
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null)
        first = newNode;
    else
        l.next = newNode;
    size++;
    modCount++;
}

linkBefore 先找到节点，再插入到前面

void linkBefore(E e, Node<E> succ) {
    // assert succ != null;
    final Node<E> pred = succ.prev;
    final Node<E> newNode = new Node<>(pred, e, succ);
    succ.prev = newNode;
    if (pred == null)
        first = newNode;
    else
        pred.next = newNode;
    size++;
    modCount++;
}

remove(int index)

unlink ，node 查找逻辑上文已经分析了

public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
}

indexOf(Object o)

就是简单的遍历记录 index 位置，找到后返回 index 值